Metal-as-Insulation HTS Insert for Very-High-Field Magnet: A Test Report After Repair

Abstract

This study is a sequel to our previous study on a 38 mm diameter cold bore metal-as-insulation (MI) HTS insert that reached 32.5 T in a 18 T background magnetic field. For refurbishing, 7 MI double-pancake (DP) coils were re-wound with used REBCO tapes to just replace the damaged HTS pieces by new ones at inner junctions. 2 DP coils were fully fabricated using new REBCO tapes. The new assembled insert was then tested under various background magnetic fields at 4.2 K. The key focuses of this paper are: 1) the damages in the DP coils; 2) their repair; 3) the characteristics resistance change of the insert; 4) the resistance value of each DP coil; 5) the maximum field of the HTS-resistive hybrid magnet; and 6) the field stability estimation of the hybrid by NMR characterization.

Numerical analysis of the contact resistance in a no-insulation layer-wound coil with a simplified electromagnetic–mechanical model

“…For an NI coil, a higher contact resistance represents a lower radial shunting current during charging and discharging, which leads to a shorter characteristic field delay time as shown in figure 7. A similar trend of contact resistance with external field was observed in experiments [19,21]. Moreover, the contact resistance may increase irreversibly after charging experiments in the high field because of the change of contact condition, as observed in [20].…”

“…The effect of contact resistance on the field delay time and quench behaviors of NI coils has been studied by a few research groups [16][17][18]. Recently, the increase of turn-to-turn contact resistance in the high field has been observed in experiments [19][20][21]. The contact resistance is a critical parameter in the development of HTS magnets, and it affects the field delay, thermal stability and total loss of the magnet [18,[22][23][24][25].…”

Rapid assessment of REBCO CC angular critical current density J c(B, T = 4.2 K, θ) using torque magnetometry up to at least 30 tesla

“…Fairly recently, a small REBCO test coil ('little big coil'-LBC) produced a magnetic field B = 14.4 T inside a B = 31.1 T water cooled resistive magnet, pushing the world record of DC field to B = 45.5 T [5]. This test showed the feasibility of building REBCO magnets well beyond 30 T. Currently, several laboratories around the world are attempting to build superconducting magnets stronger than 30 T; some of them are EMFL (30 T/40 T) [6], MIT (30.5 T) [7], Sendai (30 T) [8], RIKEN (32.5 T) [9], Hefei/Tsinghua (35 T) [10], and CAPP (30.5 T) [11]. Recently, the NHMFL has started a 40 T user magnet design project [12], building on the lessons learned from the 32 T all superconducting user magnet.…”

Metal-as-Insulation REBCO Insert: Simplified Protection Scheme and Investigation of Cooling Defect Under High-Field Operation

“…In 2019, a hybrid magnet comprised of an HTS insert and a resistive outsert successfully reached 32.5 T, resulting from the combination of 14.5 T produced by the HTS and 18 T of resistive background field [20], [21]. After this achievement, the HTS insert was refurbished, because the insert experienced two major events provoking its quench, a fault of the resistive outsert at 28 T and a thermal runaway of the HTS insert at 32.5 T [22]. Thanks to our protection scheme comprised of MI winding and a dump resistor, the windings of the inserts survived these two quenches, but the HTS joining pieces used for the inner and outer electrical junctions underwent delamination and kinks.…”

“…Since the previous tests, a 34 mm bore size SS tube was installed in the inner bore of the HTS insert for accessing the magnet center for NMR characterization or end-user sample mounting. Despite several openings, the circulation of liquid helium (LHe) inside the insert may be more difficult [22]. But more probably, a loss of cooling power has been linked to helium gas bubbles being trapped when the BdBz/dz product reaches beyond 21 T²/cm [23]- [25].…”